Recursive guessing game with patterns

guess-a-number-patterns.dart

import 'dart:io';
import 'dart:math';

final rng = Random();

sealed class State {
  const State();
}

class UninitializedState extends State {
  const UninitializedState();
}

sealed class Context {
  const Context();
}

class Output extends State {
  Output(String message) {
    print(message);
  }
}

class Input extends State {
  final String? input = stdin.readLineSync();
}

class MaybeGuess {
  MaybeGuess();

  static State from(String string) {
    try {
      final num = int.parse(string);
      return Guess(num);
    } catch (e) {
      return const InvalidInput();
    }
  }
}

class Guess extends State {
  final int number;

  Guess(this.number);
}

class InvalidInput extends State {
  const InvalidInput();
}

class Gte extends State {
  const Gte();
}

class Lte extends State {
  const Lte();
}

class Success extends State {
  const Success();
}

extension on int? {
  State toState() => this == null ? InvalidInput() : Guess(this!);
}

State program(
  int number, [
  State state = const UninitializedState(),
]) {
  try {
    final State newState = switch (state) {
      UninitializedState() => Output('Guess a number (h for help)'),
      Output() => Input(),
      Input(input: null) => Output('Enter a number'),
      Input(input: '') => Output('Enter a number'),
      Input(input: 'h') => Output('Guess a number between 0 and 100'),
      Input(input: final String input) => int.tryParse(input).toState(),
      InvalidInput() => Output('Enter a valid number'),
      Guess(number: final g) when g > number => Gte(),
      Guess(number: final g) when g < number => Lte(),
      Guess(number: final g) when g == number => Success(),
      Guess() => throw Exception('Why is this needed?'),
      Gte() => Output('Too high'),
      Lte() => Output('Too low'),
      Success() => throw Success(),
    };

    return program(number, newState);
  } on Success {
    print('You guessed it!');
    exit(0);
  } catch (e) {
    stderr.writeln(e.toString());
    exit(1);
  }
}

void main() {
  program(rng.nextInt(100));
}